In the telephone industry, modular plug type connectors are used on retractile and straight cords which are used between the base and a handset of a telephone and between the base and a wall terminal block. See, for example, U.S. Pat. No. 4,148,539 which issued Apr. 10, 1979, in the name of E. C. Hardesty.
In a modular plug, tinsel, stranded or solid conductors are confined in conductor-receiving troughs formed in a dielectric housing. Flat, blade-like metallic terminals are then inserted into individual slots in the housing in a side-by-side arrangement with contact portions thereof extending into engagement with the conductors. When the plug is inserted into a jack of a telephone handset or base or wall terminal block, portions of terminals in the jack engage portions of associated terminals in the plug.
Care must be taken so that when pulling forces are applied to cordage which is terminated by a modular plug, the forces do not appear at the connections between the conductors of the cordage and the metallic blade-like terminals. Otherwise, the electrical connections between the conductors and the terminals may be compromised. In other words, the plug must include provisions other than the electrical connections for securing the plug to the end portion of the length of cordage.
A modular plug which includes strain relief facilities which divert forces from the cordage-terminal connection points to the housing is known in the prior art. See U.S. Pat No. 3,998,514 which issued on Dec. 21, 1976, in the name of E. C. Hardesty. An anchoring member is provided in an opening of the housing and is connected to the housing through a plastic hinge oriented toward a free end of the modular plug. The anchoring member also is connected to the housing of the plug toward a cordage-input end of the housing by a frangible portion of dielectric material. The frangible portion supports the anchoring member in its initial position to facilitate insertion of an end portion of a length of cordage into the housing. Forces are applied to the anchoring member to actuate it and cause it to be moved pivotally into clamping engagement with the end portion of the cordage. The pivotal movement of the anchoring member causes originally external facing portions to be moved along the side of a wall which defines partially the opening and then to latch under a lip formed by a portion of the surface of the housing. Portions of the anchoring member and a surface which defines the opening and a surface adjacent thereto cooperate to positively hold the anchoring member in engagement with the housing and the cordage when retrograde forces are applied to the cord by the customer during use.
A problem that has surfaced relates to the engagement of the anchoring member with the jacket of the cordage. The frangible portion of the anchoring member of presently used plugs becomes embedded in the jacket of the cordage terminated by the plug. Over time, the jacket plastic flows and the degree of engagement of the anchoring member with the jacket may decrease, thereby causing some of the strain due to pulling forces applied to the cordage to be transferred to the electrical connections. Further, the embedment of the frangible portion and the geometry of the anchoring member may cause one or ones of the conductors to be broken.
Also, the problem of reduced effectiveness of the strain relief member may be exacerbated because of the number and frequency of occurrences of use of a telephone handset, for example, to which such a plug-terminated cord is connected. Such usage may decrease the time during which the flow of the plastic material occurs to the point of resulting in unacceptable loss in strain relief.
In the past, the anchoring member has been formed such that a portion of the anchoring member including the frangible portion forms a somewhat rounded trailing edge as the frangible portion is broken to allow pivotal movement of the anchoring member to engage the jacketed end portion of the cordage. Because this is the portion that is most deeply embedded into the cordage, it is most advantageous that there be no sharp burrs or edges opposed to the direction of pulling forces to tear the material comprising the jacket during customer use. Nevertheless, it has been found that because of the frequency of usage there still occurs a problem because of the nature of the embedment of the anchoring member in the cordage jacket.
Also, a modular plug of the prior art includes a conductor-restraining bar. The conductor-restraining bar is disposed in another opening which communicates with the terminal-receiving side of the housing and is supported therein by two unequal height webs of plastic material. When forces are applied, the bar separates from the housing along the webs and tends to move pivotally to engage the conductors. Such an arrangement for restraining the conductors may allow some relative movement between the conductors and the terminals and/or may result in damage to the conductors.
What is needed and seemingly what is not provided in the prior art is a modular plug having enhanced jacket and conductor strain relief provisions. Such sought after strain relief provisions must be capable of being included in the plug without having to change its present overall geometry.